Most accurate gestational age estimating by ultrasound?

[swtiepie711]

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We were taught in our OBGYN rotations that ultrasounds are most accurate for estimating GA in the 1st trimester with the rough rule of +/- 1 week in 1st tri, +/- 2 weeks in 2nd tri & +/- 3 weeks in 3rd tri.

However, I've now run into a contradiction in both UW & Secrets. Both say that dating is most accurate at 16-20 weeks with biparietal diameter (p205 in 3rd ed Secrets, don't know the Q# in UW). Secrets specifically says "When is ultraosound most accurate at estimating fetal age? At 16 to 20 weeks the biparietal diamete (measured by ultrasound) gives the most accurate estimate of fetal age." I blew it off when I saw it in UW, but then now I'm seeing it in Secrets.

In looking this up on UptoDate, it says "When the embryonic or fetal pole can be visualized, measurement of crown-rump length (CRL) is THE MOST ACCURATE METHOD OF PREGNANCY DATING (±3 days) at 7 to 10 weeks of gestation" (their caps, not mine).

So.... are Secrets & UW wrong? or I am misinterpreting something? Thanks for any insight!

 

[Knicks]

I hate situations like this.

 

[OveractiveBrain]

I call this the Reviewer's Paradox. Where do you think Secret's go their information from? Where do you think UWorld got their information from? Do you really thing they got some expert to write these questions? No, some dude read secrets and wrote a vignette for UWorld.

The earlier the ultrasound, the more diagnostic for age criteria.

The best dates is knowing implantation (i.e. the doctor does it).
The next best dates is knowing ovulation (i.e. basal body temp measurement)
The next best is LNMP and a regular cycle (i.e. may be the least reliable)
The next best date is ultrasound, earlier being better.